The interfacing of computers to peripheral devices demands that standard protocols for data transfer be established as a precondition to the easy integration of computing systems. Protocols must exist on two (2) levels: the physical and the logical. The physical level consists of the establishment of standards for the passing of electrical signals between the two devices. The logical level consists of the interpretation of the data embodied in these signals. This invention deals with the physical level.
The E.I.A. RS232 standard was originally promulgated in the early 1960's to deal with this physical data link. It established two classes of equipment; Data Termination Equipment (DTE) and Data Communications Equipment (DCE). The RS232 standard dealt with the data link between a DTE (usually either a computer or a teletype) and a DCE (usually a modem). It defined a logically bidirectional interface using mirror imaged transmit/receive pairs to transfer data in either direction. The standard defined a polarized connection utilizing twenty-five (25) conductor cable, terminated with "D subminiature" connectors. Signal levels were defined, as were minimum rise times for signals, and other electrical parameters. This standard, with its subsequent revisions, defined the physical level standard for serial communications which has become the "Lingua Franca" of data communications.
Since the establishment of the RS232 standard, numerous new classes of peripheral equipment have appeared, the polarity of which with respect to the DTE/DCE distinction is sometimes unclear. An example is the case of a personal computer with an RS232 port. Generally, these ports are configured as DTE's, in order to facilitate the connection to a modem. Occasions arise where a connection to another DTE (for example, a terminal) is desired. To accomplish this interface with current equipment, a new cable must be fabricated to "reverse" the polarity of the signals.
FIG. 1 reveals a conventional connection between a DTE and a DCE. Specifically, the diagram shows the devices labeled DCE and DTE with a simplified two signal wire connection between them. In order for data transfer to take place the transmitting port 1 of the DTE must be connected to the receiving port 2 of the DCE and vice versa. Thus, the ports of the two devices must be mirror imaged. If this is not the case (such as when a DTE is connected with another DTE), the prior art requires that a special cable be constructed which reverses the signals. The wire connection, transmitting and receiving ports together comprise a data link between the DCE and the DTE.